CLI Tool to generate Salesforce.com package.xml (and destructiveChange.xml) files based on git diff between two branches.
npm install -g sf-packager
$ sfpackage destinationBranch sourceBranch ./deploy/
This will create a package at ./deploy/sourceBranch/unpackaged/package.xml copying all files into directory.
If any deletes occurred will also create ./deploy/sourceBranch/destructive/destructiveChanges.xml
You can force a specific version for the package.xml with the -p flag
sfpackage destinationBranch sourceBranch -p 42 ./deploy/
You can also just write the package.xml and destructiveChanges.xml by passing the -d flag
sfpackage destinationBranch sourceBranch -d > ~/Desktop/packageAndDestructiveChanges.xml
You can also create "backout" content by reversing the order of the destination and source branches
sfpackage sourceBranch destinationBranch ./deploy/